I got myself into a similar situation with a hired LWB Sprinter van in a supermarket car park. The exit was a much tighter turn than the entrance. It took a while, but eventually I got the incoming traffic to stop so I could get out through the entrance. I don't know if that could have been an option in this case, but might be worth keeping in mind if anyone else found themselves in the same situation. If there's a barrier then I'd expect that someone in the supermarket would be able to open it to let you out.